Purified terephthalic acid is produced in a two stage process. In the first stage, the oxidation plant, crude terephthalic acid (CTA) is produced by the air oxidation of paraxylene in a solvent (e.g., acetic acid and water) using a homogeneous catalyst. The catalyst can be one or more heavy metal compounds (e.g., cobalt and/or manganese compounds or other heavy metals such as vanadium, chromium, iron, molybdenum, a lanthanide such as cerium, zirconium, hafnium and/or nickel and an oxidation promoter). The metal compound can take any of the forms of catalyst that have been used in the liquid phase oxidation of aromatic carboxylic acid precursor(s) in aliphatic carboxylic acid solvent (e.g., bromide, bromoalkanoates or alkanoates (usually C1-C4 alkanoates, such as acetates)). The oxidation promoter, where employed, can be in the form of elemental bromine, ionic bromide (e.g., HBr, NaBr, KBr, NH4Br) and/or organic bromide (e.g., bromobenzenes, benzyl-bromide, mono- and dibromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylene-di-bromide, etc.). Alternatively, the oxidation promoter can include a ketone (e.g., methylethylketone) or aldehyde (e.g., acetaldehyde).
The reaction temperature of about 150-210° C. or about 160-200° C. The oxidation is typically carried out in one, two or three vessels in series, and multiple reactors can be used in parallel for each step. Reactor vessels are typically agitated vessels, where the agitation is achieved by a combination of a mechanical agitation, plus the agitation effect of the air being added.
The main impurity in crude terephthalic acid is 4 carboxy benzaldehyde, a reaction intermediate and terephthalic acid precursor. This is present in the CTA at a concentration of about 1000-4000 ppm or about 2500-3500 ppm. Other intermediates and contaminants present in the CTA include paratoluic acid and organic compounds contributing to color (colored chemicals) such as 2,6 dicarboxyfluorenone. The following references describe in detail the range of colored compounds present in PTA and CTA; U.S. Pat. No. 4,626,598, U.S. Pat. No. 3,850,983, and U.S. Pat. No. 4,833,269. These colored compounds are formed in the oxidation reactor. Some of the colored compounds remain in solution and are removed from the oxidation via a byproduct purge route, but a significant proportion are precipitated in the CTA crystals and transferred in the CTA to the purification stage of the process.
The second stage of the production process is the purification of the CTA by catalytic hydrogenation in aqueous solution. Typically, CTA is dissolved in water at high pressure (70-90 bar(a)) and high temperature (275-290° C.), and hydrogenated over a fixed bed catalyst of palladium supported on carbon. In the hydrogenation process the main impurity 4 carboxy-benzaldehyde (4CBA) is reacted to paratoluic acid. Also, numerous colored compounds are hydrogenated. An example of a colored compound is 2,6 dicarboxyfluorenone, and this is converted to 2,6 dicarboxyfluorene. Whilst this has some contribution to the color in the PTA, it is less colored than 2,6 dicarboxyfluorenone present in CTA.
The resulting solution is cooled as it passes through a series of four to six crystallizers, where the majority of the paratoluic acid remains in solution and the purified terephthalic acid (PTA) is crystallized. The resulting slurry (at a temperature of 140-160° C.) is then fed to solid liquid separation device(s), such as a decanter centrifuge or rotary pressure filter. The PTA is separated from the mother liquor stream. The mother liquor stream includes 4-carboxy benzaldehyde and paratoluic acid. The mother liquor stream also contains PTA fines that slip through the separation device. These components represent a significant yield loss from the process, and it is desirable to recycle these components to the oxidation stage of the process. The mother liquor also includes hydrogenated color compounds. Approximately 50% of the hydrogenated color compounds remain with the PTA, so the remaining 50% remain in the mother liquor. Thus, methods to recycle 4-carboxy benzaldehyde and paratoluic acid and the PTA fines have been described previously (e.g., U.S. Pat. No. 5,304,676 and JP 52-128344) but these have not solved the further problems defined here.
In order to recover the paratoluic and terephthalic acids, the mother liquor must be cooled to precipitate the organic acids, which are then separated from the mother liquor. A method of producing this separation is to flash-cool the stream by reducing the pressure to atmospheric pressure, where the temperature of the mother liquor is reduced to around 100° C. Further cooling is then desirable to enable further recovery of terephthalic and paratoluic acids.